The present invention relates to optical signal transmission and, more specifically, to the use of contoured electric fields and contoured poling in polarization-independent optical waveguides for applications requiring modulation and switching of optical signals.
It is becoming increasingly important to frequently upgrade telecommunication networks to increase their capacity due to the recent rapid increase in network traffic caused by multimedia communications. Although optical technologies are replacing most transmission lines, the nodes of optical networks, such as switching and cross-connect nodes, still depend on relatively slow electrical technologies. Fore example, time-division multiplexing (TDM) systems are widely used in existing optical communications systems and are inherently dependent on electrical circuits for multiplexing and demultiplexing. As a result, the electrical nodes in these types of optical networks limit throughput.
There has been a recent shift in the industry towards an emphasis on accelerating returns on existing and upgraded networks. Companies trying to pioneer the integrated optics market, however, have run head-on into the challenge of cost-effectively integrating optical components and microelectronic technology into a single device. Accordingly, there is a need in the art for innovation in integrated component design for optical switching, modulating, attenuating, multiplexing and demultiplexing devices.
The above-noted patent application, U.S. patent application Ser. No. 09/916,238, describes in detail a number of optical waveguides designed for polarization-independent operation. One of the illustrated embodiments is directed to providing polarization independence by arranging the control electrodes and waveguide core such that appropriate poling orientations are provided in each cladding region. The waveguide achieves polarization independence by optimizing phase shifting of one dominant polarization (TE) in the first cladding region and the other dominant polarization (TM) in the second cladding region. The present invention employs similar means for providing polarization independence and presents further polarization-independent waveguide configuration. The present invention also relates to improved integrated optical devices employing the polarization-independent waveguide configurations of the present invention. Suitable waveguide devices are known in the art and are disclosed in the above-noted patent application (Ser. No. 09/916,238). 
Referring briefly to FIGS. 1 and 2, an electrooptic waveguide 10 is illustrated including first and second control electrodes 20, 22, and an optical waveguide core 30. An intersecting plane 32 normal to the surface of the waveguide core 30 and extending along the primary axis of propagation defined by the waveguide core 30 is also illustrated. For the purposes of describing and defining the present invention, it is noted that TE and TM polarized light represent two independent electromagnetic modes of an optical signal. The electromagnetic field distribution is referred to as the transverse electric (TE) mode where the electric field of the optical signal is perpendicular to the intersecting plane 32. The electromagnetic field distribution is referred to as the transverse magnetic (TM) mode where the magnetic field of the optical signal is perpendicular to the intersecting plane 32. It is also noted that in a channel waveguide of the illustrated type, the propagating modes are not purely TE or TM polarized. Rather, the modes are typically more predominantly one or the other and are commonly so designated. Accordingly, a TE polarized mode may merely comprise a distribution where the electric field component parallel to the plane of propagation is the largest component of the signal. Similarly, a TM polarized mode may merely comprise a distribution where the magnetic field component parallel to the plane of propagation is the largest component of the signal.
Specifically, in accordance with one embodiment of the present invention, an electrooptic waveguide for an optical signal is provided. The waveguide comprises a plurality of control electrodes, an optical waveguide core, and electrooptic cladding regions optically coupled to the optical waveguide core. The control electrodes are positioned to generate a contoured electric field across the cladding. The cladding is poled along a poling contour. The cladding defines an array of local TM indices of refraction nTM corresponding to the indices of refraction for the vertically oriented component TM of the optical signal in the cladding. The cladding also defines an array of local TE indices of refraction nTE corresponding to the indices of refraction for the horizontally oriented component TE of the optical signal in the cladding. The local TM indices nTM and the local TE indices nTE are each a function of a first electrooptic coefficient rPP for light parallel to a local component of the contoured electric field and a second electrooptic coefficient rlP for light perpendicular to a local component of the contoured electric field. The difference between the first and second electrooptic coefficients rPP and rlP defines an optical birefringence of an electrooptic cladding material defining the cladding. The local TM indices nTM collectively define a TM mode index of the waveguide. The local TE indices nTE collectively define a TE mode index of the waveguide. The respective orientations of the contoured electric field and the poling contour are configured to compensate for the optical birefringence of the electrooptic cladding material such that the TM mode index of the waveguide is substantially equal to the TE mode index of the waveguide.
In accordance with another embodiment of the present invention, an electrooptic waveguide for an optical signal is provided. The waveguide comprises a plurality of control electrodes, an optical waveguide core defining a primary axis of propagation, and an electrooptic cladding at least partially surrounding the core. The control electrodes are positioned to generate a contoured electric field across the cladding. The cladding is poled along a poling contour. Either the contoured electric field, the poling contour, or both are asymmetric.
In accordance with yet another embodiment of the present invention, an electrooptic waveguide for an optical signal is provided. The waveguide comprises a plurality of control electrodes, an electrooptic optical waveguide core defining a primary axis of propagation, and a cladding at least partially surrounding the core. The control electrodes are positioned to generate a contoured electric field across the core. The core is poled along a poling contour. Either the contoured electric field, the poling contour, or both are asymmetric.
In accordance with yet another embodiment of the present invention, a process is provided wherein an electrooptic waveguide is formed by: providing a waveguide substrate; positioning an optical waveguide core over a first surface of the substrate; providing a waveguide superstrate; forming at least two control electrodes on a first surface of the superstrate, wherein the control electrodes define selected electrode thicknesses; positioning a viscous electrooptic cladding material over one or both of the first surface of the substrate and the first surface of the superstrate; and urging the first surface of the waveguide substrate and the first surface of the waveguide superstrate toward each other to create a layer of cladding material between the surfaces. The cladding material, which is subsequently cured, defines a cladding material viscosity selected to permit dispersion of the cladding material about the control electrodes and the core as the first surface of the waveguide substrate and the first surface of the waveguide superstrate are urged toward each other. The cladding material is provided in a quantity sufficient to ensure that the layer of cladding material defines a cladding layer thickness at least as large as the selected electrode thicknesses.
Accordingly, it is an object of the present invention to provide improved electrode arrangements for polarization-independent waveguides, improved integrated optical devices incorporating such waveguides, and processes for manufacturing such waveguides. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.